The AAduino

News: the crowd sourcing campaign for  the AAduino will start soon, sign up at CrowdSupply to be notified! The specs have been beefed with an STM32L0 cpu and the temperature sensor is now an industrial grade TMP102.

Update: you can now order the AAduino PCB from and get a Commadorable 64 bonus PCB for free.

I have been using Nathan Chantrell’s Tiny328 for quite some time as my swiss army knife ISM radio node. Now I wanted a more slim ISM node as my setup with a Tiny328 on a breadboard is not very “deployable”. I could of course 3D print a case for the Tiny328 but I have limited access to 3D printers and do not feel I have the time to explore that exciting part of the maker world just yet. This leaves me with finding off the shelf project boxes with a compartment for 2x AA batteries and the “radioduino” (and in an acceptable form factor). That search came up disappointingly, and surprisingly, short. I did have a set of standard eBay AA battery holders and looking at the 3x variant it occured to me. I needed to shrink the radio node, and the AAduino was born.

Honey, I shrunk the radioduino!
The AAduino is an wireless Arduino clone the size of an AA battery with Keystone battery terminals rotated 180¬į to act as positive and negative terminals. It is powered by an ATMega328p and is fitted with an RFM69C companion. There is room for two DS18B20 temperature sensors and an indicator LED.

I still think it looks a bit weird :)
I still think it looks a bit weird ūüôā
It runs at 8Mhz to allow for greater life span since the CPU can run at a lower voltage. I have fused the 328 to brown out at 1.7V which is a bit out of spec at 8Mhz and slightly below what the RFM69C needs. Running at 4Mhz would be more suitable but I will see how well the node performs when the batteries are draining out.

AAduino, RFM69C side
AAduino, RFM69C side
Since the RFM69C is somewhat wider than an AA battery I used a file to make it slightly narrower. There is some room for that kind of modification without damaging the module. Update, the RFM69C will fit without modification. Next I clipped the legs of the DS18B20 until about 5mm remained and soldered it to the 3x pin header on the AAduino. I then drilled a hole in the battery box where the sensor can protrude and a small hole for the LED to shine through. The wire out of the battery box was cut, shorted and stuffed away inside the box.

AAduino inside 3xAA box
AAduino inside 3xAA box
I use battery terminals from Keystone available from RS Components, positive and negative. There seems to be a cheap eBay alternative but I have not tested those. The + and – markings on the PCB indicates (this is important, read carefully) the positive and negative poles of the battery we are pretending the AAduino is. The Keystone spring contact should be soldered to the + marking and the button contact to the – marking. There is no protection diode here so be careful. Also note! If you want to power the AAduino from a bench power supply, connect the power supply’s black negative lead to the + marking and the red positive lead to the – marking.


The 3xAA boxes are also from eBay and there seem to be two different types. One that is really good and one that is really crappy. I will let you in on the secret of buying the correct one. The good ones have a nice build quality and plastics and look like this. Note the rectangular piece of plastic below the battery compartment extending from side to side.

Good 3xAA holder
Good 3xAA holder
Looking at the crappy ones, well you can tell can’t you? In the top left corner it seems someone used a soldering iron on the poor thing. The lid does not snap in place very well and the plastics is really cheap.

Crappy 3xAA holder
Crappy 3xAA holder
Code, schematics and BOM on Github, as always.

Serializing data from IoT nodes

This is a followup to my previous post about my home automation/IoT system. I introduced some of the hardware used to build sensors and now I will look at an architecture for sending data from sensor nodes to a receiver. First I will explain the initial attempt at sending data from nodes and why that was destined to fail. Next we will look at an architecture for data serialization and quick node prototyping.


Doomed for failure

The first attempt at sending data was quite simplistic with data structures being sent through the air. The base structure looked like this:

From this struct, holding a message type and a sequence number, I defined others for different kinds of sensor data, eg. temperature:

sensor battery voltage:

plant moisture:

and so on. Whenever I thought of a new kind of node I would implement a data structure for the data being transmitted. The receiver would look at the sequence number to skip duplicate transmissions and, most importantly, the type field to know what kind of data was being sent.


This is a horrible way of doing things.


Each new node type being built would require a copy paste job on both the node side (copy a previous implementation and make changes) and one on the receiver side (again, copy previous implementation and make changes). The receiver would of course require knowledge of all different branly_report_t types.

An actual architecture

The solution? Data serialization. Taking a step back makes one realize that we have do not want system with nodes reporting temperature, other nodes reporting plant moisture and so on. We want a system with nodes transmitting data. There is quite a difference between the two you see. Frankly we do not even care what kind of data nodes transmit until we want to act on it or look at it. Discussing this problem with a colleague I came up with the following model of nodes and contacts.

  1. A node has an identification number and one or more contacts.
  2. Each contact has an index number, 0..N.
  3. A contact has a predefined type that everybody agrees on.
  4. A contact may define a fixed reporting interval.
  5. A contact is readable and possibly writable.
  6. A contact may be violated, eg. a battery contact is violated if the battery voltage drops below a defined limit.

Any transmission from the node would be a reduced to a message with an updated contact property (contact identification number, contact value and flags for violations). A writable contact (eg. an RGB light) would receive messages with the same kind of data. Lots and lots of contact types can be defined and adding more further down the road is not as much of a hassle as the copy-and-past jobs mentioned earlier. Even more important, a node can report any data we want it to. My garage node could report inside and outside temperatures as well as the light intensity level (needed to know if I forgot to turn off the lights at night) without me cooking up some strange data structure only implemented once in one single node.

An example node

Before diving into how this idea of data serialization is implemented, let us look at an example. The Arduino node sketch below uses the RFM69 ISM radio to send data and has two contacts; battery voltage (reported every 1h 30min) and temperature (reported every 15min).

Yup, it does not take more code than that to create the full node software stack. Define the node, its contacts with contact getters (and setters if contact is writeable), define reporting intervals and call;


A node has, besides its contacts, version fields for hardware and software and an internal state (refer to BranlyNode.cpp). Upon boot the node step though its mCurState state variable until it has synchronized with the receiver. It will go through the following steps, requiring an ACK from the receiver in each step:

  1. send a ping packet and wait for a response from the receiver.
  2. send a hello packet with HW/SW versions.
  3. send a complete list of its contacts.
  4. send a complete contact report.
  5. wait until the next reporting interval and send a report on that specific contact

The receiver requires no configuration in order to receive data from a node. Instead, the node will tell the receiver what kind of data it will transmit.


A contact (see BranlyContact.cpp) has an index and type as mentioned as well as a getter function and optional reporting interval. Contacts may be “enqueued” meaning its report will be transmitted on the next call to A button contact will typically be¬†enqueued from an interrupt handler.

The radio protocol

The radio protocol has beed designed to be minimalistic to save power (critics may say the ping packet is not strictly necessary) and the heavy lifting is done in BranlyProtocol.cpp.

Each contact in the contact list (see buildContactListPacket) occupies a single byte, 1 bit for writeable, 3 bits for index and 4 bits for type (currently limiting the number of contacts to 8 per node and contact types to 16 in all, probably needs to be increased).

In the contact report (see buildContactReportPacket), contacts are encoded in 2-5 bytes with a flag field (for tracking violations), a size field telling how many bytes are needed to encode the current contact value and the actual value (1-4 bytes).

Lastly, contact value reports (see buildContactValuePacket) also uses 2-5 bytes and are sent in the same manner as contact reports.

The receiver

The receiver reads frames tom the BranlyPi modem running this receiver sketch. On the host side, the receiver is written in python and posts the received data to an Emoncms installation. The beauty of it all is that the receiver can create the Emoncms feed for each contact for a node on the fly based on the contact list packet.



Closing remarks

The current architecture has really enabled me to quickly prototype new nodes without changing a single line of code in the receiver and very few lines of code are needed to create a new RF node. On the receiver side, names need to be added for nodes and contacts enabling automatic bridging between RF node updates and, say, MQTT. Martin Harizanov is working on something similar and it will be exciting to see what he has come up with. If you know other implementations on this topic, please let me know in the comments below.


Meet the Branly IoT platform

These days it seems everybody is tinkering with their own IoT project, I am no different.

It started almost two years ago when I, while looking to build RF nodes, stumbled across the blogs of Nathan Chantrell, Martin Harizanov, Jean-Claude Wippler and Felix Rusu. These guys had built exactly what I was looking for and Nathan even offered the gerbers needed to order Tiny328 PCBs. Tiny328 is an Arduino based RF node fitted with the RFM69C ISM radio. My experience with soldering was limited and my experience with designing PCBs was none, I am a software guy after all. It turned out that reflow soldering PCBs with 0603 components in a hot plate is quite easy (and fun!) once you get the hang of it. Google is your friend.

My fist baby steps in designing PCBs was an Arduino based modem with the RFP69 radio and though hole components, the BranlyPi v2 (v1 was without an external oscillator which caused the ATMega’s buadrate to be off leaving the modem and the RaspberryPi unable to talk).

It was superseeded by the v4 (don’t ask about the v3) which was surface mounted components all the way. Schematics and gerbers are here¬†if you want to build your own.

With all that hardware available it was time to write some software and make two Tiny328s talk. The “hello world” was LowPowerLab‘s “Struct send” and “Struct receive” examples.¬†I soon attached a DS18B20 temperature probe to one of the nodes for some more fun (sic!) data. Software for the BranlyPi was written as well as a python script running on the RaspberryPi listening to the BranlyPi modem.

In all, I had the following setup:

  1. A node fills in data into a struct holding a type field. The types can be “temperature”, “battery voltage measurement” and so on. The struct is transmitted to the BranlyPi modem.
  2. The BranlyPi modem receives the struct and formats in into a textual representation that is sent to the RaspberryPi over UART, eg.

  3. The gateway script on the RaspberryPi reads the line above from the UART and posts it to a local Emoncms installation.

The node was placed in my garage with the probe outside. I had (sort of) built my own RF thermometer. Cool! An identical node (with a different node id ūüôā was built and placed indoor. Next, how about building one of those moisture measuring plant nodes? Or one that could tell me if I had forgotten to turn off the lights in the garage at night? I had a feeling the RF network would grow.

So far all is well from a technical point of view. The current software architecture would however need a complete rewrite to be useful, more of that later. Source code and hardware docs on Github.

“Branly” you might ask? √Čdouard Branly¬†was one of the early pioneers in wireless telegraphy.

My wifi ghost lamp

Sometime ago all wifi hobbyist projects where quite expensive. USB Wifi dongles could be found for a handfull or dollars apiece but something easily connectible to a micro controller was also “connected” with a hefty price tag. Then the ESP8266 surfaced…

Having just purchased a Pac Man ghost LED lamp on the Black Friday sale I quickly saw an application for the ESP8266. Nothing novel, nothing that hadn’t been done 100 times before but something fun.


Make the ghost green
A warranty waiting to be voided

The software part

Following in the MQTT footsteps of previous projects I decided against running a server on the ESP8266 listening to yet another protocol implementation. Instead the ghost would listen to an MQTT topic and light up accordingly. I also wanted it to be able to run preconfigured light shows.¬†The ESP8266 runs tuanpmt’s MQTT implementation and listens to the topic ghost/led sending anything it receives to the Arduino which responds to the following commands:

  • #RRGGBB¬†– set all LEDs to specified color
  • :nnRRGGBB¬†– set LED nn to specified color
  • !¬†– update the LED ring with all LEDs set with the :nnRRGGBB commands
  • p01 – run “light show” 01
  • * – turn all LEDs off

The “anything it receives” means I can add functionbality on the Arduino without touching¬†the firmware on the ESP8266.

The hardware part

The hardware consists of the following:

  • ESP8266-01 board
  • Arduino Nano
  • 16 LED Neopixel Ring
  • Level shifter
  • 3.3V regulator with buddy capacitors
  • 1A USB charger

I purchased an Adafruit Neopixel ring with the lovely WS2812 LEDs. To save some time I¬†took an Arduino¬†I had lying around to control the LEDs using the Adafruit library.¬†The two systems talks over UART. The Arduino runs at 5V (the level at which it can control the Neopixel ring) and the ESP8266 run at 3.3V which means a level shifter needs to be placed between the two. You can find level shifters almost for free on eBay. The wiring¬†is quite simple. The USB charger provides 5V for the Arduino, the Neopixel ring and the high side side of the level shifter. The regulator provides¬†3.3V for the ESP8266 and the low side of the level shifter. I threw everything together on a prototyping board, nothing will be visibe anyhow ūüôā

Once wired up the ghost can be tested with the command

The hard part

The board sits on nylon spacers to be glued to the lamp base
The board sits on nylon spacers to be glued to the lamp base

Now for the hard part, mounting the hardware inside the ghost. The ghost case is made up of three parts. The dome, the base and a bottom plate where the electronics is attached. The bottom plate is attached to the base using screws and the base is glued to the dome. This leaves the screws on the inside meaning the ghost cannot be cracked open without causing permanent damage. Using a fine saw and a drill I removed the bottom plate, sanded the edges and rinsed the dome in water ro remove the plastic dust. The electronics is mounted on spacers on the bottom plate and I used silicone to reattach it to the dome. It stays in place and reopening should not be that hard.

More software

Controlling the ghost via the command line would not attract much venture capital so I set out to¬†make an iPhone app for controlling the ghost. NKOColorPickerView¬†and MQTTKit¬†fit the bill and resulted in this neat little app I installed on my son’s iPad.

Make the ghost green
Make the ghost green

He was somewhat impressed and have changed the ghost color about three times to date which was in line with my expectations. I do have other plans for the wifi ghost, but that’s for another post.

Code available on Github

Final thoughts

In the time between finishing this hack and actually writing about it, Arduino on the ESP8266 has matured phenomenally so today I would skip the AVR. Actually I could replace all the hardware with the WifiPixels.